Lithographic apparatus and device manufacturing method

ABSTRACT

A liquid supply system for an immersion lithographic apparatus provides a laminar flow of immersion liquid between a final element of the projection system and a substrate. A control system minimizes the chances of overflowing and an extractor includes an array of outlets configured to minimize vibrations.

This application is a continuation of U.S. patent application Ser. No.11/098,615, filed Apr. 5, 2005, which is incorporated herein itsentirety by reference.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see for example U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate using a liquidconfinement system (the substrate generally has a larger surface areathan the final element of the projection system). One way which has beenproposed to arrange for this is disclosed in WO 99/49504, herebyincorporated in its entirety by reference. As illustrated in FIGS. 2 and3, liquid is supplied by at least one inlet IN onto the substrate,desirably along the direction of movement of the substrate relative tothe final element, and is removed by at least one outlet OUT afterhaving passed under the projection system. That is, as the substrate isscanned beneath the element in a −X direction, liquid is supplied at the+X side of the element and taken up at the −X side. FIG. 2 shows thearrangement schematically in which liquid is supplied via inlet IN andis taken up on the other side of the element by outlet OUT which isconnected to a low pressure source. In the illustration of FIG. 2 theliquid is supplied along the direction of movement of the substraterelative to the final element, though this does not need to be the case.Various orientations and numbers of in- and out-lets positioned aroundthe final element are possible, one example is illustrated in FIG. 3 inwhich four sets of an inlet with an outlet on either side are providedin a regular pattern around the final element.

Another solution which has been proposed is to provide the liquid supplysystem with a seal member which extends along at least a part of aboundary of the space between the final element of the projection systemand the substrate table. Such a solution is illustrated in FIG. 4. Theseal member is substantially stationary relative to the projectionsystem in the XY plane though there may be some relative movement in theZ direction (in the direction of the optical axis). A seal is formedbetween the seal member and the surface of the substrate. Desirably theseal is a contactless seal such as a gas seal. Such a system with a gasseal is illustrated in FIG. 5 and disclosed in EP-A-1 420 298 herebyincorporated in its entirety by reference.

In EP-A-1 420 300 the idea of a twin or dual stage immersion lithographyapparatus is disclosed. Such an apparatus is provided with two stagesfor supporting the substrate. Leveling measurements are carried out witha stage at a first position, without immersion liquid, and exposure iscarried out with a stage at a second position, where immersion liquid ispresent. Alternatively, the apparatus has only one stage.

The seal member disclosed in EP-A-1 420 298 has several problems.Although the system can provide immersion liquid between the finalelement of the projection system and the substrate, the immersion liquidcan sometimes overflow and sometimes recirculation of immersion liquidin the space between the final element of the projection system and thesubstrate occurs which can result in imaging errors when the radiationbeam is projected through the recirculation areas thereby heatingimmersion liquid up and changing its refractive index. Furthermore,overflow of the seal member is hard to avoid in certain circumstances.

SUMMARY OF THE INVENTION

It is desirable to provide a seal member or barrier member whichovercomes some of the above mentioned problems. It is an aspect of thepresent invention to provide a seal member or barrier member in whichturbulent flow is reduced and in which overflowing of the immersionliquid is reduced.

According to an aspect of the present invention, there is provided alithographic apparatus including a substrate table constructed to hold asubstrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate, and a barriermember having a surface surrounding a space between a final element ofthe projection system and the substrate table configured to contain aliquid in the space between the final element and the substrate; thebarrier member including a liquid inlet configured to provide liquid tothe space and a liquid outlet configured to remove liquid from thespace; wherein the liquid inlet and/or liquid outlet extend(s) around afraction of the inner circumference of the surface.

According to another aspect of the present invention, there is provideda lithographic apparatus including a substrate table constructed to holda substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate, and a barriermember having a surface surrounding a space between a final element ofthe projection system and the substrate table configured to contain aliquid in the space between the final element and the substrate; thebarrier member including a liquid inlet configured to provide liquid tothe space, the inlet including a chamber in the barrier member separatedfrom the space by a plate member, the plate member forming at least partof the surface and having a plurality of through holes extending betweenthe chamber and the space for the flow of liquid therethrough.

According to another aspect of the present invention, there is provideda lithographic apparatus including a substrate table constructed to holda substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; a liquid supplysystem configured to supply liquid to a space between a final element ofthe projection system and a substrate; and a control system configuredto dynamically vary the rate of extraction of liquid by the liquidsupply system from the space and/or dynamically vary the rate of supplyof liquid by the liquid supply system such that a level of liquid in thespace is maintained between a predetermined minimum and a predeterminedmaximum.

According to another aspect of the present invention, there is provideda lithographic apparatus including a substrate table constructed to holda substrate; a projection system configured to project a patternedradiation beam onto a target portion of the substrate; and a liquidsupply system configured to provide liquid to a space between a finalelement of the projection system and a substrate; wherein the liquidsupply system includes an extractor configured to remove liquid from thespace, the extractor including a two dimensional array of orificesthrough which the liquid can be extracted from the space.

According to another aspect of the present invention, there is provideda device manufacturing method including projecting a patterned beam ofradiation onto a substrate using a projection system, wherein a barriermember has a surface which surrounds the space between a final elementof the projection system which projects the patterned beam and thesubstrate thereby containing a liquid in a space between the finalelement and the substrate; providing liquid to the space through aliquid inlet; and removing liquid from the space via a liquid outlet,wherein the liquid inlet and/or liquid outlet extend(s) around afraction of the inner circumference of the surface.

According to another aspect of the present invention, there is provideda device manufacturing method including projecting a patterned beam ofradiation onto a substrate using a projection system, wherein a liquidis provided between a final element of the projection system and thesubstrate, the liquid being contained by a barrier member having asurface, the liquid being provided to the space through an inlet whichincludes a chamber in the barrier member separated from the space by aplate member and the plate member having a plurality of through holesextending between the chamber and the space through which the liquidflows.

According to another aspect of the present invention, there is provideda device manufacturing method including projecting a patterned beam ofradiation onto a substrate using a projection system, wherein liquid isprovided to a space between the final element of a projection system andthe substrate and the rate of extraction of liquid from the space isdynamically varied and/or the rate of supply of liquid to the space isdynamically varied to maintain the level of liquid in the space betweena predetermined minimum and a predetermined maximum.

According to another aspect of the present invention, there is provideda device manufacturing method including projecting a patterned beam ofradiation onto a substrate using a projection system, wherein liquid isprovided to a space between a final element of a projection system and asubstrate; liquid being extracted from the space through an extractorwhich includes a two dimensional array of orifices.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe present invention;

FIGS. 2 and 3 depict a liquid supply system used in a prior artlithographic projection apparatus;

FIG. 4 depicts a liquid supply system according to another prior artlithographic projection apparatus;

FIG. 5 depicts a seal member as disclosed in European Application No.03252955.4;

FIG. 6 depicts schematically, in cross-section, a seal member of thepresent invention;

FIGS. 7 a and b depict, in plan, a seal member of the present invention;

FIGS. 8 a-c depict variations in flow direction through the seal memberwith hole diameter to plate thickness ratio of immersion liquid;

FIGS. 9 a-e illustrate different embodiments of overflows according tothe present invention;

FIGS. 10 a-e depict different embodiments for liquid extractionaccording to the present invention; and

FIG. 11 depicts the control system for the management of immersionliquid in the seal member according to the present invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the present invention. The apparatus includes anillumination system (illuminator) IL configured to condition a radiationbeam B (e.g. UV radiation or DUV radiation). A support (e.g. a masktable) MT is constructed to support a patterning device (e.g. a mask) MAand is connected to a first positioning device PM configured toaccurately position the patterning device in accordance with certainparameters. A substrate table (e.g. a wafer table) WT is constructed tohold a substrate (e.g. a resist-coated wafer) W and is connected to asecond positioning device PW configured to accurately position thesubstrate in accordance with certain parameters. A projection system(e.g. a refractive projection lens system) PS is configured to project apattern imparted to the radiation beam B by patterning device MA onto atarget portion C (e.g. including one or more dies) of the substrate W. Areference frame RF is configured to support the projection system PS.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support supports, e.g. bears the weight of, the patterning device.It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus, and other conditions, such as for example whether or not thepatterning device is held in a vacuum environment. The support can usemechanical, vacuum, electrostatic or other clamping techniques to holdthe patterning device. The support may be a frame or a table, forexample, which may be fixed or movable as required. The support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives radiation from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation is passed from the source SO tothe illuminator IL with the aid of a beam delivery system BD including,for example, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the lithographic apparatus,for example when the source is a mercury lamp. The source SO and theilluminator IL, together with the beam delivery system BD if required,may be referred to as a radiation system.

The illuminator IL may include an adjusting device AD to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support (e.g., mask table MT), and ispatterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which projectsthe beam onto a target portion C of the substrate W. With the aid of thesecond positioning device PW and a position sensor IF (e.g. aninterferometric device, linear encoder or capacitive sensor), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the radiation beam B.Similarly, the first positioning device PM and another position sensor(which is not explicitly depicted in FIG. 1 but which may be aninterferometric device, linear encoder or capacitive sensor) can be usedto accurately position the mask MA with respect to the path of theradiation beam B, e.g. after mechanical retrieval from a mask library,or during a scan. In general, movement of the mask table MT may berealized with the aid of a long-stroke module (coarse positioning) and ashort-stroke module (fine positioning), which form part of the firstpositioning device PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioning device PW. In the case of a stepper,as opposed to a scanner, the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

FIG. 6 illustrates the seal member or barrier member 12 of the presentinvention. Working radially outwardly from the optical axis of theprojection system, there is provided a plurality of inlets 124 throughwhich immersion liquid 500 is provided to the space 11 between theprojection system PS and the substrate W. On the bottom surface 80 ofthe seal member 12 there is then provided a liquid removal device 180such as the one disclosed in U.S. application Ser. No. 10/921,348 filedAug. 19, 2004, hereby incorporated in its entirety by reference.Radially outwardly of the liquid removal device 180 is provided a recess320 which is connected through inlet 322 to the atmosphere and viaoutlet 324 to a low pressure source. Radially outwardly of recess 320 isa gas knife 420. The arrangement of these three items on the bottomsurface 80 of the seal member 12 is described in detail in U.S.application Ser. No. 60/643,626 filed Jan. 14, 2005 hereby incorporatedin its entirety by reference. At the top inner surface of the sealmember 12 is a vertically extending protrusion or dike 220 over whichimmersion liquid 500 can flow into overflow area 222 and which can thenbe extracted through hole array 224 via a low pressure source attachedto port 228.

FIG. 6 is a schematic cross-section of the seal member 12. Each of thefive elements described above are not necessarily present around theentire circumference of the seal member. This is particularly the casewith the immersion liquid inlets 124 and the liquid outlet or extractor(i.e. the dike 220/hole array 224). As can be seen in FIG. 7 a, thesecan be advantageously provided only around a localized innercircumference of the seal member 12 and desirably opposite each other.As can be seen from figures, the liquid inlets 124 and liquid outlet areat different distances from the substrate W. Suitable fractions oflength of liquid inlets 124 and/or liquid outlet is less than ½,desirably less than ⅓ of the inner circumference of the seal member 12.Desirably the length of the liquid inlets 124 and/or liquid outlet ismore than 1/20, more desirably more than 1/15 or 1/10 of the innercircumference of the seal member 12. This helps in creating a laminarnon-turbulent flow of immersion liquid from the outlets 124, across thespace 11 (i.e. a cross-flow) between the projection system PS and thesubstrate through the target portion TP through which the radiation beamimages the substrate, and out of the space through hole array 224. It isalso possible to encourage flow of the immersion liquid across the space11 by providing the liquid extraction unit 180 on the opposite side ofthe seal member 12 to the inlet ports 124 but this is not necessarilythe case. Alternatively, the extraction unit 180 can be positionedaround the entire circumference, perhaps with a larger extractionpressure applied to it opposite the inlets 124. FIG. 7 b illustratesanother embodiment in which three liquid outlets or extractors 224 areprovided around the inner circumference of the barrier member 12. Thethree outlets are positioned at roughly 120° apart, with the biggestoutlet being opposite to the liquid inlets 124 and the other two outletsbeing smaller and positioned on either side of the inlets 124.

The way in which the liquid is provided to the liquid inlets 124 and thedesign of the liquid inlets 124 themselves will now be described indetail with reference to FIGS. 6 and 8. As can be seen in FIG. 6,immersion liquid is provided through inlet 128 into the seal member 12.A first pressure drop is created in the immersion liquid by forcing itthrough an orifice 121 which puts a first chamber 120 into liquidcommunication with a second chamber 122. In reality orifice 121 is aplurality of individual holes created in plate 123, separating thechambers 120 and 122. The plurality of holes 121 are arranged in aregular one-dimensional array in the illustrated embodiment, but otherarrangements such as two or more parallel rows of holes 121 one aboveanother can also be used. The holes 121 distribute the flow over plate126, which separates chamber 122 from the space 11, in the tangentialdirection and ensure a homogeneous flow over the whole width of thearray of orifices 124 irrespective of the configuration of the supply128. Once the immersion liquid has entered the second chamber 122, itenters through orifices 124 into the space 11 between the projectionsystem PS and the substrate W. The orifices 124 are provided in a(regular) two-dimensional array in the plate 126 of the seal member 12.This creates a parallel, homogeneous flow inside the space 11. The arrayof orifices 124 is positioned towards the lower surface 80 of the plate126, desirably below the level of the projection system PS when the sealmember 12 is in use.

The present inventors have found that the ratio of orifice 124 diameterd to outer plate 126 thickness t may be considered in controlling thedirection in which the immersion liquid leaves the chamber 122. This iseven the case if all of the orifices 124 are drilled through the plate126 in a plane which will be parallel to the substrate W in use.

As can be seen from FIG. 8 a, if the diameter d of the orifice 124 isgreater than the thickness t of the outer plate 126, the flow ofimmersion liquid can exit at an angle illustrated by arrow 127 i.e. nonparallel to the substrate W surface. In FIG. 8 b, the wall thickness tis equal to the diameter d of the orifice 124 and in FIG. 8 c, thediameter d of the orifice 124 is less than the thickness t of the outerwall 126. It has been found that the orifice diameter should be lessthan the thickness of the plate 126. Typically the plate thickness willbe of the region of 0.4 mm and the diameter of the orifice 124 is in theregion of 0.15 mm for flow to exit parallel to the substrate surface andparallel to the direction in which the orifice is machined in the plate126 (the plate 126 is not necessarily vertically orientated and can beinclined as illustrated). The dimensions are a trade off between havingsmall enough orifices 124 to create a large enough pressure drop andhaving a plate thickness thick enough to give the desired stiffness. Asa result, a much more laminar flow with a lower velocity and less mixingis produced than with prior art designs. The parallel flow is encouragedby making the small orifice in a relatively thick plate. The desiredratio of plate thickness t to orifice diameter d is at least 1:2.5 sothat the flow can be directed in the same direction as the axis of theorifice. The orifices are machined (drilled) substantially parallel toeach other and substantially parallel to the plane of the substrate Wand substantially perpendicular to the surface of the plate 126 throughwhich they extend. The orifices can be cut by laser as small as 20 μmand as large as desired. Another way of manufacturing small holes in aplate is by electroforming (electrolytical deposition) of, for example,nickel. Holes with a diameter of 5 to 500 μm in a sheet of thicknessbetween 10 μm and 1 mm are possible using this technique. This techniquecan be used to produce both inlets and outlets as described elsewhere inthis description. However, unlike with the other manufacturing methods,it is difficult to align accurately the axis of the through hole usingthis method.

It has been found that the number of orifices and the angle their axismakes with the outer plate 126 as well as their diameter has an effecton the direction in which the liquid flows. Generally, with a singlehole, flow is directed slightly away from the axis of the hole towardsthe side of the plate with which the axis of the hole makes an acuteangle, i.e. in FIG. 8, if the axis of the hole is parallel to thesubstrate W, slightly downwards from horizontal towards the substrate.The more holes that are present, the more pronounced the effect. Thiseffect can be used to redirect flows of any fluid types in manyapplications (e.g. airshowers, purge hoods) and thereby eliminate orreduce the need for vanes or deflection plates or use of the Coandaeffect. The effect is so strong that it can act against the force ofgravity. It is thought that the origin of the effect is the interactionof a large number of asymmetrical fluid jets. The flow deflection alsooccurs when the fluid flows into a large volume of the same fluid, sothe flow deflection is not related to the teapot leakage problem wheretea leaks along the spout of the teapot. If the outer wall 126 isvertical, the axis of the orifices 124 should be parallel to thesubstrate W upper surface. If the outer wall 126 is included, asillustrated, in order to achieve flow parallel to the substrate surface,it has been found that the axis of the orifices 124 should be inclinedaway from the top surface of the substrate by about 20 degrees,desirably in the range of from 5 to 40 degrees.

The two-step pressure drop (there is a pressure drop as described, whenthe liquid goes through orifices 121 and clearly there will also be apressure drop when the liquid passes through orifices 124) is arrangedto be over the whole of the width of the supply and height of thesupply. In this way the first pressure drop ensures that the flow isprovided evenly over the orifices 124 irrespective of the supply channelconfiguration (i.e. the channel between input 128 and chamber 120), asdescribed.

The laminar flow is desirable because it prevents recirculation ofimmersion liquid which can result in those recirculated areas of liquidbecoming hotter or colder than the remaining liquid and therefore havinga different refractive index or resulting in certain areas of the resistbeing more dissolved by the immersion liquid than others (i.e. anon-uniform concentration of resist in the immersion liquid which canchange the refractive index of the immersion liquid) and also preventingtransport of the resist to the projection lens.

Desirably the density of holes in the plate 126 is of the order of 15holes per square mm. A range of from 1 to 30 holes per square mm isdesirable.

In prior art seal members, liquid has been extracted either from thebottom surface 80 of the seal member 12 or from a single outletpositioned in the inner wall of the seal member 12 defining the space11. The outlet has either been a one dimensional array of holes aroundthe entire circumference of the inner surface of the seal member 12 orhas been an annular groove around the circumference. A problem with thistype of liquid extraction is that the holes in the inner wall of theseal member are either extracting or are not extracting and thetransition between extraction and non extraction can result inundesirable vibrations of the seal member 12. One solution which hasbeen proposed is disclosed in European Patent Application No.04256585.3, hereby incorporated in its entirety by reference. In thatdocument, a dike 220 is provided similar to the one illustrated in FIG.6. Here, if the level of immersion liquid 500 in the space rises abovethe level of the dike, it overflows the dike into a pool or overflow 220behind the dike and with a lower level than the dike. The immersionliquid may then be removed from the overflow 222. Again a difficultywith this system is that extraction either tends to happen or does nothappen and there is a difficulty with the control of the amount ofextraction resulting in occasional overflow.

In the present invention, a two dimensional array of holes or mesh 224is provided in a wall of the seal member 12 through which liquid isextracted. Immersion liquid which either overflows a dike 220 or flowsabove the level of the lower most hole of the 2 d array 224 is extractedby extractor 228. Desirably a non-homogenous array of holes in the wallof the seal member is used in which the number of holes per unit areaand/or size of holes increases from a minimum furthest away from thesubstrate to a maximum nearest the substrate or at lowest position. Thusthere is a smaller resistance for the immersion liquid to pass throughthe array at the lowest level and a higher resistance for air at theupper level of the plate. Thus by using a vertical gradient in the holedistribution (either in size or density or both) the resistance of theplate to flow is increased with increasing vertical height. Thus theproblem of the flow of air out through the holes pushing away water andthereby making level control difficult is addressed. Such embodimentsare illustrated in FIGS. 9 a-e. In an alternative embodiment illustratedin FIGS. 10 a-e no dike is present and the immersion liquid is removedas soon as its level reaches above the lower most hole of array 224. Asis illustrated in FIGS. 7 a and b, the extraction arrangementsillustrated in FIGS. 9 a-e and 10 a-e may be provided only around afraction of the inner circumference of the seal member 12, desirablyopposite the inlets 124. However, clearly the outlets illustrated inFIGS. 9 a-e and 10 a-e can be provided the whole way around the innercircumference of the seal member 12. It is possible to provide adifferent level of under pressure to the outlet 228 around thecircumference of the seal member in the latter embodiment therebyarranging for different extraction rates around the inner circumferenceof the seal member 12. Arranging for different extractions rates eitherby varying the pressure of an extractor extending around the entirecircumference of the seal member 12 or by arranging for only a localizedextractor can help in promoting laminar flow of immersion liquid fromthe inlets 124 across the target portion TP and out through theextractor.

The array of holes 224 may include holes of the order of between 0.1 and0.5 mm in diameter. A density of 0.25 to 5 holes per square mm isdesirable. The use of the two dimensional array of holes has the benefitthat the immersion liquid 11 is more easily controlled because a higherimmersion liquid level wets more holes of the array 224 resulting in ahigher extraction rate. Conversely, a lower level of immersion liquidwill wet fewer holes and thereby result in a lower extraction rate. Inthis way the extraction of immersion liquid is automatically regulatedwithout the need for adjusting the extraction rate at outlet 228. Thisis particularly the case when the hole array 224 is vertically or atleast partly vertically orientated. The use of a dike 220 allows thearray of holes 224 to extend to a lower level than the dike increasingthe extraction capacity. If the barrier member 12 is made liquid philic(hydrophilic in the case that the immersion liquid is water) build up ofliquid level due to surface tension effects can be minimized.

The overflow 220 allows sudden and short build-up of immersion liquidwithout the risk of over spilling. For example, during moving of thesubstrate W or a closing disc up closer to the surface of the sealmember 12 there will be a sudden decrease in the volume of the space 11and therefore a rise in immersion liquid level. The ditch 222 canaccommodate some of this excess liquid while it is extracted.

It should be appreciated that the array of holes 310 could be providedas a mesh or equivalent.

FIGS. 9 a-e illustrate different configurations for the dike embodimentof the extractor. In FIG. 9 a, the immersion liquid enters a volume 330before being extracted by extractor 228. By contrast, in the design ofFIG. 9 b, it is arranged that the immersion liquid enters a narrow gap340 before being extracted at outlet 228. Due to capillary forces, thegap 340 is completely filled with immersion liquid (if it is designednarrow enough) and if the under-pressure is matched with the size of theholes 224 the formation of bubbles in the extracted immersion liquid orthe inclusion of bubbles in the extracted immersion liquid can beprevented thereby making the extraction flow a single phase flow andthereby preventing deleterious vibrations. In FIGS. 9 c and 9 d,different angles of the wall in which the array of holes 224 are formedare illustrated. In FIG. 9 e a top plate 223 is added above the overflowarea which enhances the extraction capacity due to the fact that thesuction of the liquid is brought closer to the projection system PS,where the liquid meniscus tries to follow the projection system contour.The purpose of these diagrams is to illustrate that many configurationsare possible which still have the aspects of the present invention.

FIGS. 10 a-e illustrate various embodiments without the dike 220. Anyangle of inclination of the wall in which the array of holes 224 areformed is possible and different configurations of paths for theimmersion liquid to follow to the outlet 228 are illustrated. Forexample, in FIG. 10 b, the gap 340 is similar to the gap in FIG. 9 bsuch that single phase flow extraction is possible, in FIG. 10 e, thetop plate 223 is similar to that in FIG. 9 e.

Another way to help minimize the risk of overflow of immersion liquid isillustrated in FIG. 11. The system illustrated in FIG. 11 matches theamount of incoming immersion liquid with the amount of removed immersionliquid by dynamically varying the rates of extraction and input. As canbe seen, immersion liquid is supplied to the seal member 12 throughinlet 128 and is removed through outlets 184, 228 and 328 as isdescribed above in reference to FIG. 6. Having a controllable supplyallows more flexibility in operating circumstances. For example, morevariations in the leak flow rate through outlet 328 are allowable andeven if the extraction system 224 does not have sufficient capacity tocope with the maximum flow, that does not necessarily lead to overflowbecause the supply of immersion liquid can be reduced to compensate.Even with a constant supply flow, a controllable extraction is desirablebecause different operating conditions, for example scanning in adifferent direction, can result in variable leak or extractionparameters which can be coped with by varying the extraction. Eachextraction port includes a controllable valve 1228, 1184, 1328. Theoutlet ports 228, 128, 328 are all connected to a low pressure sources2228, 2148, 2328 via a valve as illustrated. Extracted immersion liquidis lead to a reservoir 1500 which, if the immersion liquid is to berecycled, can be the source for the inlet 1248. The supply is controlledby a valve 1128 and an overflow path to the reservoir 1500 is providedwith a valve 1148 controlling that.

The water level control mechanism allows the supply rate of immersionliquid to be varied as well as the extraction through the overflow 224,through the liquid extractor 180 and through the recess extractor 320.Each of the valves 1228, 1148, 1128, 1184, 1328 are variable valvesthough they may be valves which are either on or off. The amount ofextraction can be varied either by varying the under pressure applied,using the valves controlling the under pressure or by varying the valves1128, 1184, 1328 or by varying the bypass to ambient (also illustratedin FIG. 11).

There are three options to determine when a dynamic control action isneeded. These are direct feedback in which the level of the immersionliquid is measured, indirect feedback in which the extraction flows fromeach of the extractors is measured or feed-forward in which a knowledgeof the extraction flow and the operating circumstances is used to adjustthe supply and/or extraction flows when circumstances change.

The water level may be measured in several ways, for example by a floatin the reservoir 1500 or in the space 11, or by measuring the pressureof water at the bottom of the seal member 12. By determining theposition of the water surface by reflection and detection of acousticalor optical signals on the upper surface of the immersion liquid. Furtherpossibilities are by measuring the absorption or transmission of anacoustical, optical or electrical signal as a function of the amount ofwater or by measuring heat loss of a submerged wire in a known positionin the space 11, the further the wire is submerged, the higher the heatloss.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. It should be appreciated that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a metrology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itshould be appreciated that the present invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the present invention have been describedabove, it will be appreciated that the present invention may bepracticed otherwise than as described. For example, the presentinvention may take the form of a computer program containing one or moresequences of machine-readable instructions describing a method asdisclosed above, or a data storage medium (e.g. semiconductor memory,magnetic or optical disk) having such a computer program stored therein.

The present invention can be applied to any immersion lithographyapparatus, in particular, but not exclusively, those types mentionedabove.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic apparatus, comprising: a substrate table constructedto hold a substrate; a projection system configured to project apatterned radiation beam onto a target portion of the substrate; aliquid supply system configured to supply liquid to a space between theprojection system and the substrate table; and a control systemconfigured to dynamically vary a rate of extraction of liquid by theliquid supply system from the space and/or dynamically vary a rate ofsupply of liquid by the liquid supply system such that a level of liquidin the space is maintained between a certain minimum and a certainmaximum.
 2. The lithographic apparatus of claim 1, wherein the controlsystem is configured to dynamically vary the rate or rates in responseto a determination of a level of liquid in the space.
 3. Thelithographic apparatus of claim 2, further comprising a pressure sensorconfigured to measure a pressure of the liquid at a certain position inthe space to determine the level of liquid in the space.
 4. Thelithographic apparatus of claim 2, further comprising an optical and/oracoustic source and a corresponding optical and/or acoustic detectorconfigured to determine the level of liquid in the space by reflectionand subsequent detection of an optical and/or acoustic signal off thetop surface of the liquid.
 5. The lithographic apparatus of claim 2,further comprising an acoustical/optical/electrical signal generatorconfigured to generate an acoustical/optical/electrical signal in liquidin the space and a detector configured to detect theacoustical/optical/electrical signal to determine the level of liquid inthe space.
 6. The lithographic apparatus of claim 2, further comprisinga wire configured to be submerged in the liquid at a certain location inthe space and a detector configured to measure a temperature of the wireto determine the level of liquid in the space.
 7. The lithographicapparatus of claim 2, further comprising a float configured to float onthe top surface of the liquid in the space and a sensor configured tomeasure a position of the float to determine the level of liquid in thespace.
 8. The lithographic apparatus of claim 1, wherein the controlsystem is configured to actively vary the rate or rates based on ameasurement of an amount of liquid extracted from the space by theliquid supply system.
 9. The lithographic apparatus of claim 1, whereinthe control system is configured to dynamically vary the rate or ratesin a feed forward manner based on operating circumstances of theapparatus.
 10. The lithographic apparatus of claim 1, further comprisingvalves configured to control the rate of extraction and/or supply. 11.The lithographic apparatus of claim 1, further comprising valvesconfigured to control an under pressure applied to a liquid extractor ofthe liquid supply system.
 12. A device manufacturing method, comprising:projecting a patterned beam of radiation onto a substrate using aprojection system of a lithographic apparatus, wherein liquid isprovided to a space between the projection system and the substrate anda rate of extraction of liquid from the space is dynamically variedand/or the rate of supply of liquid to the space is dynamically variedto maintain a level of liquid in the space between a certain minimum anda certain maximum.
 13. A lithographic apparatus, comprising: a substratetable constructed to hold a substrate; a projection system configured toproject a patterned radiation beam onto a target portion of thesubstrate; and a barrier member having a surface surrounding a spacebetween the projection system and the substrate table, the barriermember being configured to at least partly confine a liquid in thespace, the barrier member comprising a liquid inlet configured toprovide liquid to the space and a liquid outlet configured to removeliquid from the space, wherein the liquid inlet and/or the liquid outletextends around a fraction of the inner periphery of the surface.
 14. Thelithographic apparatus of claim 13, wherein the fraction is less thanabout ½.
 15. The lithographic apparatus of claim 13, wherein thefraction is less than about ⅓.
 16. The lithographic apparatus of claim13, wherein the fraction is more than about 1/20.
 17. The lithographicapparatus of claim 13, wherein the fraction is more than about 1/15. 18.The lithographic apparatus of claim 13, wherein the liquid inlet and theliquid outlet are positioned on the surface such that they face oneanother across the space.
 19. The lithographic apparatus of claim 13,wherein the liquid inlet and liquid outlet are positioned alongdifferent parts of the surface around the inner periphery.
 20. Thelithographic apparatus of claim 13, wherein the liquid outlet isarranged to provide a variable liquid extraction rate along its lengthin the direction following the inner periphery.
 21. The lithographicapparatus of claim 20, wherein a maximum extraction rate is providedsubstantially opposite the liquid inlet.
 22. The lithographic apparatusof claim 13, wherein the liquid outlet extends substantially around theinner periphery.
 23. The lithographic apparatus of claim 13, wherein theliquid inlet and the liquid outlet extend around a fraction of the innerperiphery of the surface, the fraction of the inner periphery for theliquid inlet being smaller than the fraction of the inner periphery forthe liquid outlet.
 24. The lithographic apparatus of claim 13, whereinthe liquid outlet is positioned radially outwardly, relative to theoptical axis of the projection system, of the liquid inlet.
 25. Thelithographic apparatus of claim 13, comprising at least three liquidoutlets, one liquid outlet facing the liquid inlet across the space andone liquid outlet on each side of the liquid inlet.
 26. A devicemanufacturing method, comprising: projecting a patterned beam ofradiation onto a substrate using a projection system, wherein a barriermember has a surface which surrounds a space between the projectionsystem and the substrate, the barrier member configured to at leastpartly contain a liquid in the space; providing liquid to the spacethrough a liquid inlet; and removing liquid from the space via a liquidoutlet, wherein the liquid inlet and/or the liquid outlet extends arounda fraction of the inner periphery of the surface.